It is well known that coordination anionic polymerization using catalyst systems represented by a Ziegler-Natta catalyst allows for homopolymerization of olefins and dienes. However, it was difficult to provide efficient copolymerization of olefins and dienes using such polymerization systems.
To overcome this difficulty, for example, JP 2000-154210 A (PTL 1) discloses a catalyst for polymerizing conjugated dienes that contains a transition metal compound of group IV of the periodic table having cyclopentadiene ring structure, and also refers to an α-olefin such as ethylene as an exemplary monomer copolymerizable with this conjugated diene. However, PTL 1 does not provide a specific description of copolymerization of a conjugated diene compound and a non-conjugated olefin. Obviously, there is no description or suggestion of manufacturing a rubber excellent in crack growth resistance, heat resistance and ozone resistance by controlling the cis content and cis-1,4 bond content to be greater than 92%.
For example, JP 2006-249442 A (PTL 2) discloses a catalyst for polymerizing olefins that consists of a transition metal compound such as a titanium compound and a co-catalyst, and also discloses a copolymer of an α-olefin and a conjugated diene compound. However, specific manufacture and use were ensured only if the α-olefin, a non-conjugated olefin, is contained in an amount of 66.7 mol % to 99.1 mol %. That is, there is no description or suggestion in PTL 2 of manufacturing a rubber excellent in crack growth resistance, heat resistance and ozone resistance by controlling the cis content and cis-1,4 bond content to be greater than 92%.
In addition, JP 2006-503141 A (PTL 3) discloses a copolymer of ethylene and butadiene that is obtained by synthesizing ethylene and butadiene as a starting material by means of a special organic metal complex as a catalytic component. However, PTL 3 states that butadiene, a monomer, is inserted into the copolymer in the form of transformer-1,2-cyclohexane, and the structure disclosed in PTL 3 is different from that of the copolymer of the present invention. In addition, specific manufacture and use were ensured only if the ethylene, a non-conjugated olefin, is contained in an amount of 69.6 mol % to 89.0 mol %. In this case, the ethylene content was determined by 100 mol % minus the molar content of those units derived from butadiene with a known molar content. That is, there is no description or suggestion in PTL 3 of manufacturing a rubber excellent in crack growth resistance, heat resistance and ozone resistance by controlling the cis content and cis-1,4 bond content to be less than 92%.
In addition, JP 2000-086857 A (PTL 4) discloses a butadiene polymer having cis content of 92%, vinyl content of 6% and ethylene content of 3% or 9%. However, there is no description or suggestion in PTL 4 of manufacturing a rubber excellent in crack growth resistance, heat resistance and ozone resistance by controlling the cis content and cis-1,4 bond content to be greater than 92%.
In addition, JP 2000-154279 (PTL 5) discloses a rubber composition including: a butadiene-ethylene block copolymer having a cis content of 92% and ethylene segments in an amount of 4.8 mass % of the total; polybutadiene having a cis content of 95.2% and a vinyl content of 2.5%; and carbon black N220. However, there is no description or suggestion in PTL 5 that a rubber excellent in crack growth resistance, heat resistance and ozone resistance can be obtained by controlling the cis content and cis-1,4 bond content to be greater than 92%.
Further, JP 11-228743 A (PTL 6) discloses an unsaturated elastomer composition that is composed of an unsaturated olefin-based copolymer (an olefin-rich, olefin-diene copolymer) and rubber. However, there is no description or suggestion in PTL 6 of the cis content and vinyl content of the olefin-diene copolymer, even of manufacture of a rubber excellent in crack growth resistance, heat resistance and ozone resistance by controlling the cis content and cis-1,4 bond content to be greater than 92%.
Additionally, in response to increasing social demands for energy and resource saving, there is an increasing need for a rubber material that is excellent in wear resistance and crack growth resistance to meet the requirements for improving durability of tires in order to reduce fuel consumption of automobiles. In addition, in view of the recent surge in price of butadiene, it is expected that the price of raw materials will even more dramatically rise in the future. As such, there is a need to make use of inexpensive olefin resource even in tire materials.
To address this issue, attempts have been made conventionally to combine high cis-butadiene rubber with natural rubber. There is a problem, however, in that sufficient wear resistance cannot be obtained by combining these rubbers due to incompatibility between them.
In addition, the characteristics required when a rubber composition containing a copolymer of a conjugated diene and a non-conjugated olefin is applied to various applications (such as tires, conveyor belts and anti-vibration rubber) include good wear resistance and crack growth resistance.
For example, PTL 1-6 disclose the aforementioned techniques, but fail to teach or suggest that a rubber which is excellent in wear resistance and crack growth resistance may be obtained by mixing a conjugated diene-based polymer with a conjugated diene compound/non-conjugated olefin copolymer in which the cis-1,4 bond content of a unit derived from the conjugated diene compound is more than 92%.